I liken emulation to having a box and blocks of wood of different shapes. The Pandora is the box, and the blocks of wood are a puzzle that needs to fit in that box. Things like NES and Gameboy are small puzzles: you can see that the blocks will easily fit in the box, you could practically just dump them in without actually solving the puzzle. SNES and Genesis are a little bigger: you need to do at least some of the puzzle in order to get it small enough to fit, but many of the pieces can just be dumped in anyway, and if you do enough of the puzzle you have room left over for extra features. N64 and Playstation you see this big pile of blocks and can imagine that there might be some way to arrange them so they'll just fit, maybe a little wiggle room, but not much else. Dreamcast and PSP is even a little bigger pile than that: if the puzzle can be built into the box, it will just barely fit with no room for error.
Saturn, PS2, etc... they have a huge pile of pieces; an expert puzzle builder looks at it and knows that it doesn't matter how you arrange them, they just won't fit in the box.